If you look at the ingredients for Alka-Seltzer, you will find that it contains citric acid and sodium bicarbonate (baking soda). When you drop the tablet in water, the acid and the baking soda react -- this produces the fizz.
Shake and Bake A new and increasingly widespread technique also known as one pot, this condenses the anhydrous method's many steps and requires only a plastic soda bottle, the size of which depends on the quantity of meth the cook intends to make.
Do You Have a Problem? A new method of manufacturing methamphetamine, designed to get around laws restricting sale of the ingredients needs to make meth, is spreading across the country and law enforcement officials claim the new shake and bake process is even more dangerous than the old makeshift meth labs.